This document discusses operating system I/O systems. It covers I/O hardware including devices, ports, buses and controllers. It describes how operating systems manage I/O through techniques like interrupts, DMA, blocking/non-blocking I/O, buffering and caching. The kernel I/O subsystem handles requests, scheduling, error handling and protection. Interfaces like STREAMS provide communication between processes and devices. I/O performance is important to overall system performance.

1. OPERATING SYSTEM
Chapter 12: I/O Systems

2. Chapter 13: I/O Systems
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I/O Hardware
Application I/O Interface
Kernel I/O Subsystem
Transforming I/O Requests to Hardware Operations
STREAMS
Performance

3. Objectives
• Explore the structure of an operating system’s I/O
subsystem
• Discuss the principles of I/O hardware and its complexity
• Provide details of the performance aspects of I/O
hardware and software

4. Overview
• I/O management is a major component of operating system design and
operation
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Important aspect of computer operation
I/O devices vary greatly
Various methods to control them
Performance management
New types of devices frequent
• Ports, busses, device controllers connect to various devices
• Device drivers encapsulate device details
– Present uniform device-access interface to I/O subsystem

5. I/O Hardware
• Incredible variety of I/O devices
– Storage
– Transmission
– Human-interface
• Common concepts – signals from I/O devices interface with computer
– Port – connection point for device
– Bus - daisy chain or shared direct access
– Controller (host adapter) – electronics that operate port, bus, device
• Sometimes integrated
• Sometimes separate circuit board (host adapter)
• Contains processor, microcode, private memory, bus controller, etc
– Some talk to per-device controller with bus controller, microcode, memory, etc

6. A Typical PC Bus Structure

7. I/O Hardware (Cont.)
• I/O instructions control devices
• Devices usually have registers where device driver places
commands, addresses, and data to write, or read data from
registers after command execution
– Data-in register, data-out register, status register, control register
– Typically 1-4 bytes, or FIFO buffer
• Devices have addresses, used by
– Direct I/O instructions
– Memory-mapped I/O
• Device data and command registers mapped to processor address space
• Especially for large address spaces (graphics)

8. Device I/O Port Locations on PCs (partial)

9. Polling
• For each byte of I/O
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Read busy bit from status register until 0
Host sets read or write bit and if write copies data into data-out register
Host sets command-ready bit
Controller sets busy bit, executes transfer
Controller clears busy bit, error bit, command-ready bit when transfer done
• Step 1 is busy-wait cycle to wait for I/O from device
– Reasonable if device is fast
– But inefficient if device slow
– CPU switches to other tasks?
• But if miss a cycle data overwritten / lost

10. Interrupts
• Polling can happen in 3 instruction cycles
– Read status, logical-and to extract status bit, branch if not zero
– How to be more efficient if non-zero infrequently?
• CPU Interrupt-request line triggered by I/O device
– Checked by processor after each instruction
• Interrupt handler receives interrupts
– Maskable to ignore or delay some interrupts
• Interrupt vector to dispatch interrupt to correct handler
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Context switch at start and end
Based on priority
Some nonmaskable
Interrupt chaining if more than one device at same interrupt number

11. Interrupt-Driven I/O Cycle

12. Intel Pentium Processor Event-Vector Table

13. Interrupts (Cont.)
• Interrupt mechanism also used for exceptions
– Terminate process, crash system due to hardware error
• Page fault executes when memory access error
• System call executes via trap to trigger kernel to execute request
• Multi-CPU systems can process interrupts concurrently
– If operating system designed to handle it
• Used for time-sensitive processing, frequent, must be fast

14. Direct Memory Access
• Used to avoid programmed I/O (one byte at a time) for large data
movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between I/O device and memory
• OS writes DMA command block into memory
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Source and destination addresses
Read or write mode
Count of bytes
Writes location of command block to DMA controller
Bus mastering of DMA controller – grabs bus from CPU
When done, interrupts to signal completion

15. Six Step Process to Perform DMA Transfer

16. Application I/O Interface
I/O system calls encapsulate device behaviors in generic classes
Device-driver layer hides differences among I/O controllers from kernel
New devices talking already-implemented protocols need no extra work
Each OS has its own I/O subsystem structures and device driver
frameworks
• Devices vary in many dimensions
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Character-stream or block
Sequential or random-access
Synchronous or asynchronous (or both)
Sharable or dedicated
Speed of operation
read-write, read only, or write only

17. A Kernel I/O Structure

18. Characteristics of I/O Devices

19. Characteristics of I/O Devices (Cont.)
• Subtleties of devices handled by device drivers
• Broadly I/O devices can be grouped by the OS into
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Block I/O
Character I/O (Stream)
Memory-mapped file access
Network sockets
• For direct manipulation of I/O device specific characteristics,
usually an escape / back door
– Unix ioctl() call to send arbitrary bits to a device control register
and data to device data register

20. Block and Character Devices
• Block devices include disk drives
– Commands include read, write, seek
– Raw I/O, direct I/O, or file-system access
– Memory-mapped file access possible
• File mapped to virtual memory and clusters brought via demand paging
– DMA
• Character devices include keyboards, mice, serial ports
– Commands include get(), put()
– Libraries layered on top allow line editing

21. Network Devices
• Varying enough from block and character to have own interface
• Unix and Windows NT/9x/2000 include socket interface
– Separates network protocol from network operation
– Includes select() functionality
• Approaches vary widely (pipes, FIFOs, streams, queues,
mailboxes)

22. Clocks and Timers
• Provide current time, elapsed time, timer
• Normal resolution about 1/60 second
• Some systems provide higher-resolution timers
• Programmable interval timer used for timings, periodic interrupts
• ioctl() (on UNIX) covers odd aspects of I/O such as clocks and
timers

23. Blocking and Nonblocking I/O
• Blocking - process suspended until I/O completed
– Easy to use and understand
– Insufficient for some needs
• Nonblocking - I/O call returns as much as available
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User interface, data copy (buffered I/O)
Implemented via multi-threading
Returns quickly with count of bytes read or written
select() to find if data ready then read() or write() to transfer
• Asynchronous - process runs while I/O executes
– Difficult to use
– I/O subsystem signals process when I/O completed

24. Two I/O Methods
Synchronous
Asynchronous

25. Kernel I/O Subsystem
• Scheduling
– Some I/O request ordering via per-device queue
– Some OSs try fairness
– Some implement Quality Of Service (i.e. IPQOS)
• Buffering - store data in memory while transferring between devices
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To cope with device speed mismatch
To cope with device transfer size mismatch
To maintain “copy semantics”
Double buffering – two copies of the data
• Kernel and user
• Varying sizes
• Full / being processed and not-full / being used
• Copy-on-write can be used for efficiency in some cases

26. Device-status Table

27. Sun Enterprise 6000 Device-Transfer Rates

28. Kernel I/O Subsystem
• Caching - faster device holding copy of data
– Always just a copy
– Key to performance
– Sometimes combined with buffering
• Spooling - hold output for a device
– If device can serve only one request at a time
– i.e., Printing
• Device reservation - provides exclusive access to a device
– System calls for allocation and de-allocation
– Watch out for deadlock

29. Error Handling
• OS can recover from disk read, device unavailable, transient
write failures
– Retry a read or write, for example
– Some systems more advanced – Solaris FMA, AIX
• Track error frequencies, stop using device with increasing frequency of
retry-able errors
• Most return an error number or code when I/O request fails
• System error logs hold problem reports

30. I/O Protection
• User process may accidentally or purposefully attempt to
disrupt normal operation via illegal I/O instructions
– All I/O instructions defined to be privileged
– I/O must be performed via system calls
• Memory-mapped and I/O port memory locations must be protected too

31. Use of a System Call to Perform I/O

32. Kernel Data Structures
• Kernel keeps state info for I/O components, including open
file tables, network connections, character device state
• Many, many complex data structures to track buffers,
memory allocation, “dirty” blocks
• Some use object-oriented methods and message passing to
implement I/O
– Windows uses message passing
• Message with I/O information passed from user mode into kernel
• Message modified as it flows through to device driver and back to process
• Pros / cons?

33. UNIX I/O Kernel Structure

34. I/O Requests to Hardware Operations
• Consider reading a file from disk for a process:
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Determine device holding file
Translate name to device representation
Physically read data from disk into buffer
Make data available to requesting process
Return control to process

35. Life Cycle of An
I/O Request

36. STREAMS
• STREAM – a full-duplex communication channel between a user-level
process and a device in Unix System V and beyond
• A STREAM consists of:
- STREAM head interfaces with the user process
- driver end interfaces with the device
- zero or more STREAM modules between them
• Each module contains a read queue and a write queue
• Message passing is used to communicate between queues
– Flow control option to indicate available or busy
• Asynchronous internally, synchronous where user process communicates
with stream head

37. The STREAMS Structure

38. Performance
• I/O a major factor in system performance:
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Demands CPU to execute device driver, kernel I/O code
Context switches due to interrupts
Data copying
Network traffic especially stressful

39. Intercomputer
Communications

40. Improving Performance
• Reduce number of context switches
• Reduce data copying
• Reduce interrupts by using large transfers, smart controllers,
polling
• Use DMA
• Use smarter hardware devices
• Balance CPU, memory, bus, and I/O performance for highest
throughput
• Move user-mode processes / daemons to kernel threads

41. Device-Functionality Progression

42. End of Chapter 12